Add missing resizeMode prop on Image Android

Summary:
It seems like the `resizeMode` propType was missing from `Image.android.js`, this PR adds it.
I caught by trying to use `Image.propTypes.resizeMode` on Android, and getting a warning because it was `undefined`.

It's not supposed to break anything, maybe pop a few warnings but not much more.

**Test Plan:**
- Launched UIExplorer
Closes https://github.com/facebook/react-native/pull/7355

Differential Revision: D3648559

fbshipit-source-id: a4397938d14c11ded909d6bf4652a4b0fcebed3b

Libraries/Image/Image.android.js

@@ -130,6 +130,22 @@ var Image = React.createClass({
      * Used to locate this view in end-to-end tests.
      */
     testID: PropTypes.string,
+    /**
+     * Determines how to resize the image when the frame doesn't match the raw
+     * image dimensions.
+     *
+     * 'cover': Scale the image uniformly (maintain the image's aspect ratio)
+     * so that both dimensions (width and height) of the image will be equal
+     * to or larger than the corresponding dimension of the view (minus padding).
+     *
+     * 'contain': Scale the image uniformly (maintain the image's aspect ratio)
+     * so that both dimensions (width and height) of the image will be equal to
+     * or less than the corresponding dimension of the view (minus padding).
+     *
+     * 'stretch': Scale width and height independently, This may change the
+     * aspect ratio of the src.
+     */
+    resizeMode: PropTypes.oneOf(['cover', 'contain', 'stretch']),
   },
 
   statics: {